Introduction of the hectopascal in the UK

The hectopascal was introduced into radiotelephony usage and meteorological reports and forecasts in the UK, and this change was effective from 0001 UTC 17 November 2011.

The millibar is directly equivalent to the hectopascal (hPa). No conversion factor is required as the only difference is the name. CAA documents and publications will be amended to adopt the hectopascal in lieu of the millibar for measurement of atmospheric pressure as the publications go through their normal amendment cycles. Some documents will be amended in advance of the introduction into operational usage.

In radiotelephony transmissions in the UK, the word millibars is appended to pressure values of less than 1000 mb, or in cases where confusion or ambiguity may result, eg. '997 millibars'. This is to ensure that pilots, who routinely use inches as the unit of atmospheric pressure, do not confuse a millibar setting with a setting in inches. 'Millibars' may be omitted for values greater than 999 mb. This convention will be continued with the adoption of hectopascal and used in the same manner.

Note: No abbreviation of hectopascal will be used in R/T phraseology. See CAP 413 (link below) for full details.


Yellow AIC: (Y 055/2011 Released 14-JUL-2011) Downloadable from the NATS website.

CAP 413 (latest edition) is available on the CAA website

Atmospheric Picture

FL45, English Channel.